I. Field of the Invention
This invention is concerned with improvements in or relating to methods of mounting circuit components on flexible substrates and, in particular, surface mounting components to the conductors of printed circuits on flexible substrates having relatively low temperature heat resistance.
One well known method of mounting circuit components on rigid substrates in the manufacture of printed circuits is referred to as surface mounting. In this method, the components are positioned in the desired location on the surface of the printed circuit substrate and connected to the conductors of the circuit by reflow soldering techniques which are well known in the manufacture of printed circuits. In reflow soldering, the printed circuit substrate and the components are exposed to relatively high temperatures in order to activate the solder paste. It has also been proposed to surface mount components on flexible printed circuit substrates but, in so doing, care must be taken to ensure that the substrate is sufficiently resistant to the reflow soldering temperatures. Many film materials, including polyesters, have not been used satisfactorily for surface mounting components because of inadequate heat resistance and dimensional stability. In practice, only flexible circuit substrates of polyimide film have been found to be suitable for surface mounting of components, in view of the resistance of polyimide films to the temperatures encountered in reflow soldering.
Flexible printed circuit substrates are used extensively for some purposes, usually where a rigid substrate cannot be easily accommodated in the space available. One common use of flexible printed circuits is for vehicle instrument clusters but these circuits have been relatively simple and have not required surface mounting of components.
Most of these simple flexible printed circuits, especially for use in automobile instrument clusters, have used flexible polyester film to provide the flexible substrate. Flexible polyester films are much cheaper than polyimide films, approximately one quarter or one fifth of the price of polyimide films, but polyester films have much lower softening points than polyimide and, in general, are unstable at the temperatures commonly found in reflow soldering.
II. Related Art